Silica bodies, whether porous, non-porous or superficially porous, are used as solid supports for the construction of stationary phases in liquid chromatography. There are many processes for producing such silica bodies. Methods for producing porous bodies include crushing and grading silica and precipitation from sodium silicate solutions. Other processes for preparing porous microspheres are described in U.S. Pat. No. 3,857,924, issued to Halasz et al, U.S. Pat. No. 4,131,542, issued to Bergna et al, U.S. Pat. No. 4,477,492 issued to Bergna et al.
Another process for preparation of a highly desirable porous silica for use in chromatography is described by Iler et al. in U.S. Pat. No. 4,010,242. The process described is a coacervation process in which silica sol particles and a urea-formaldehyde polymer form micron-size liquid droplets. These droplets when hardened, are harvested and fired at high temperature to burn out the organic binder leaving behind porous silica microspheres. This firing process also serves to sinter the silica sol particles to their immediate neighbors, thereby building up the particle aggregate and strengthening the aggregate. Silica microspheres prepared by such a procedure are available from E. I. du Pont de Nemours & Co., Inc., (Wilmington, DE) under the tradename Zorbax.RTM..
The silica microspheres of Iler et al. have the desirable properties of being mechanically very strong, having a very narrow particle diameter range, a narrow pore size distribution and a uniform pore distribution. Mechanical strength is desirable to resist crushing during column packing or use. Narrow particle diameter range and pore and pore size distribution are desirable properties for efficient size exclusion chromatography.
Unfortunately, silica microspheres of Iler et al. have the undesirable properties of having a dehydroxylated surface, limited porosity [Unger et al., J. Chromatog., Volume 296, 3 (1984) and Knox et al., J. Chromatog., Volume 185, 289 (1979)] and an acidic surface [Engelhardt et al., J. Chromatog., Volume 218, 395 (1981) and Kohler et at., J. Chromatog., Volume 352, 275 (1986)]. By dehydroxylation is meant the elimination of surface silanol groups with concommitant formation of siloxane groups. This dehydroxylation results from the thermal treatment to burn out the organic polymer. This is undesirable as the silanol groups are the groups to which bonded phases, such as silanes, are attached. The limited porosity also results from the thermal treatment which also serves to sinter the microspheres to achieve the desired mechanical strength. As higher temperatures and longer treatment times are used, the porosity of the resulting particles decreases. This is particularly undesirable in size exclusion chromatography where porosity has a direct effect upon resolution. The acidic surface may also result from the sintering process. The nature of the acidic sites on the surface is not known, but may be due to cationic impurities or pyrolyzed carbon remaining on the surface. An acidic surface is undesirable in that it leads to non-symmetrical peak shapes during chromatography of basic compounds [Kohler et al., J. Chromatog., Volume 352, 275 (1986)].
A variety of processes are known for cleaning and rehydroxylating silica surfaces. Nestrick et al. (U.S. Pat. No. 4,376,641 issued Mar. 15, 1983) disclose a hot acid leaching process for cleaning and rehydroxylating the inner column surface of glass capillary tubes for use in gas chromatography. The cleaning aspects of this treatment are said to remove undesirable cationic species from the surface. This hot acid leaching process is reported to be used following a surface etching procedure. The surface etching is accomplished with methanolic solutions of KHF.sub.2 or NH.sub.4 HF.sub.2 (ammonium bifluoride).
A variety of processes are known for increasing the pore size of silica bodies. Vespalec et al. [J. Chromatogr., Volume 354, 129-143 (1986)] report that specific pore volume of silica can be increased by treatment with phosphate ion containing solutions. Control of the increase in porosity is not taught.
Keston (U.S. Pat. No. 3,485,687 issued Dec. 23, 1969) disclose a process for increasing the pore size of porous glass by treating with ammonium fluorides, such as ammonium bifluoride, and mineral acid at elevated temperature. The mineral acid treatment is said to release HF in situ thereby dissolving the glass and increasing the pore size. The ammonium bifluoride is reported to be ineffective when used alone.
Filbert et al. (U.S. Pat. No. 3,453,806 issued July 8, 1969) describe the use of aqueous HF or ammonium bifluoride to surface leach non-porous alkali-silicate beads to increase the surface available for binding to a liquid solvent useful in gas chromatography. It is suggested that leaching roughens or grooves the surface of the beads. They report that control of the leaching process is achieved by selection of the particular reagent, the strength of the leaching solution, the temperature of the solution and the length of treatment time.
Strauss et al. (U.S. Pat. No. 3,650,960 issued Mar. 21, 1972) describe the use of a mixture of HF and ammonium fluoride to etch silicon dioxide for use in semi-conductor devices. Kirkland in a patent application, Ser. No. 798,333, filed Nov. 1, 1985, prepares microspheres for chromatographic application according to a process comprising contacting heat strengthened thermally-dehyroxylated porous silica miscrospheres having a total concentration of silanol groups of less than about 5.5 .mu.mol/m.sup.2 with water in the presence of HF or at least one basic activator selected from the group consisting of quaternary ammonium hydroxides, ammonium hydroxide, and organic amines at a temperature of from about ambient temperature to about 100.degree. C. for sufficient time to generate a desired concentration of silanol groups.
Despite all of this background knowledge, there remains a need for an improved method of increasing the porosity of silica bodies in a controlled way and also cleaning the surface of said silica bodies.